Electric discharge devices



Oct. 4, 1955 G. F. KLEPP ELECTRIC DISCHARGE DEVICES Filed May 29, 1952 F/GS.

Inventor GEORGE F. KLE PP By 0%, r Attorney United States Patent ELECTRIC DISCHARGE DEVICES George F. Klepp, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 29, 1952, Serial No. 290,619

Claims priority, application Great Britain June 15, 1951 7 Claims. (Cl. 313-174) The present invention relates to cold cathode gas filled electric discharge tubes for use as relays in electrical switching and counting applications.

Cold cathode trigger tubes have now long been known in which the main discharge path between an anode and a cathode is triggered by means of a discharge between an auxiliary trigger electrode and the cathode, the breakdown voltage of the trigger gap being considerably less than that of the main gap. In such tubes the cathode has commonly been activated by coating it with electron emissive material so as to reduce the work function of the cathode surface and hence to enable lower operating voltages to be used than if a clean metal cathode were provided. These activated cathode tubes, however, particularly those for small discharge current operation, have heretofore suffered from a number of defects such as instability of characteristics with life, undue sensitivity to ambient illumination and discrepancies between D. C. and pulse operation. It is today possible to produce cold cathode tubes using unactivated cathodes which are very reliable and stable in operation and have characteristics which are reproducible from sample to sample. Due to their lower operating voltages, however, tubes with activated cathodes remain in demand. The present invention is concerned primarily with the construction of a small general purpose triode tube suitable for use with a 200 volt supply line and to pass a discharge current of the order of ().Sl.5 ma. through the main gap.

In order to reduce the effects of discharges collected on the glass envelope of a discharge tube and also, in some measure, the effects of ambient illumination of the cathode surface, we have previously proposed to construct trigger tubes with the electrodes arranged so that when the tube is mounted in a normal upright position a cathode disc or cup activated on its lower surface is located above an anode rod sealed in the envelope base. In such designs the trigger electrodes are normally also rods, similar to the anode, but closer to the cathode surface.

According to the present invention there is provided a cold cathode gas filled electric discharge tube comprising an anode opposed to a cathode having an activated discharge surface of extended area forming with the said anode a main discharge gap and a trigger elec trode adjacent the said cathode forming therewith an auxiliary discharge gap of lower striking voltage than the said main gap, wherein the said cathode is annular and the said trigger electrode is positioned Within the enclosure of the cathode, the auxiliary discharge gap being substantially at right angles to and adjacent the discharge path of the main gap.

An embodiment of the invention will be described with reference to the accompanying drawings in which:

Fig. 1 shows an elevational view of a tube according to the invention.

Fig. 2 shows an enlarged perspective view of the electrode arrangement within the tube of Fig. 1.

2,719,933 Patented Oct. 4, 1 955 Fig. 3 shows a plan view of the electrode arrangement of Fig. 2.

Fig. 4 shows a plan view of a cathode before assembly with the other electrodes of Fig. 2 and Fig. 5 shows a cross section of the cathode taken along the line AA of Fig. 4.

Referring now to Fig. 1, the tube there shown has an envelope 1 closed at one end by a glass press 2 and having a tubulation 3 at the other end. Three leads 4, 5 and 6 are shown sealed in line in the glass press. The tube being of small dimensionsabout 1 /2 inches long by /2 inch in diameter-it may be mounted in similar manner to components such as resistors rather than being provided with pins for engagement with a normal type of valve socket, circuit connections being made directly to I the said leads which may be identified by cutting them to different lengths, the lead 4 being shown in Fig. l shorter than the leads 5 and 6. The leads 4, 5 and 6 are sutliciently rigid to be used as electrode supports; inside the envelope they continue as rods 7, 8 and 9 respectively. The middle rod 8 is surrounded along the lower part of its length by a glass sleeve 10, and the exposed cylindrical portions of the three rods are oxidised to prevent them becoming activated by particles sputtered from the cathode. The outer rod 7 is shorter than the other two rods and its upper end 11, which is cut off square, forms the anode of the main gap. The other outer rod 9 forms a conducting support for an annular cathode 12 through which the middle rod 8 protrudes, a getter assembly 13 being welded thereto above the cathode. A metal strip forming a trigger electrode 14 is welded to the rod 8 and positioned so as to be broadside on to the inner edge of cathode 12 above the anode 11.

The electrode arrangement is more clearly shown in Fig. 2. The under surface of cathode 12 is coated with electron emissive material, the upper side being calorised.

, cathode we should normally arrange the trigger electrode I so that the auxiliary discharge between cathode and trigger occurred at a well defined point on the cathode immediately above the main anode; in the case of an activated cathode, however, due to slight differences in the work function of the coated surface, it does not follow that the path of lowest breakdown voltage is necessarily the shortest distance between the electrodes. Furthermore, if for some time the main discharge gap has been passing a small current which does not fully cover the available cathode surface with cathode glow, the unused portions of the cathode surface tend to develop a higher work function than those portions which have previously been supplying electrons. In order, therefore, to ensure constancy of operating characteristics during life, we find it necessary to use a trigger electrode having an extended discharge surface area opposed to the cathode; the metal strip forming the trigger electrode 14, therefore, is formed to follow an arc coaxial with and opposing the interior edge of the cathode. The trigger-cathode arrangement shown on the drawings has been found in practice to give very good transfer characteristics, while as the trigger electrode itself is not between the cathode and the main anode 11, it does not divide the discharge area of the cathode into two halves, while its surface condition plays little part in the characteristics of the main discharge gap.

As is seen from the plan View of the electrode system shown in Fig. 3, the inner and outer cathode edges are eccentric to one another, so that the cathode is of greater width at 15, above the anode 11, than at other parts. This greater width ensures that with small discharge currents which occupy only a portion of the cathode surface there is a bias tending to maintain the discharge at the part of the cathode adjacent to the trigger electrode and nearest to the anode.

In the present embodiment the cathode 12 is an annular disc which may be stamped out from a metal strip leaving a tag 16 at the narrowest portion of the disc. This tag is bent over and welded to the rod 9. In Fig. 4 the cathode is shown as it appears before the tag 16 is bent over. In order to ensure that the trigger discharge may readily transfer from the cathode edge to the main cathode anode gap opposite anode 11, the inner surface of the cathode is bevelled on the side facing the main gap as indicated at 17 in Figs. 4 and 5. During manufacture of the tube, after the electrodes have been assembled within the envelope, the tube is pumped, the glass is baked to drive off occluded gases, the cathode is activated, and the getter is fired by eddy current heating, after which the tube is filled with a mixture of 94% neon and 6% argon at 70 mm. Hg. It should be noted that the getter 13 is positioned in the tube so as to minimise the risk of getter material contaminating any of the electrode discharge surfaces. After sealing off, the final activation and ageing is carried out.

Typical performance data for a tube according to the present invention is as follows:

Breakdown voltage of main gap volts 240 Main gap maintaining voltage do 73 Trigger gap .breakdown voltage do 80 Trigger gap maintaining voltage do 65 Cathode current ma 1.0

With the anode supplied from a source of 180 volts through a series resistance of 100,000 ohms, a square trigger pulse of 90 volts amplitude and 100 ,usec. width applied between trigger electrode and cathode through a 1 megohm resistor in series with the trigger electrode causes the main gap to fire on or before the trailing edge of the trigger pulse. A negative pulse of rectangular shape reducing the anode voltage to zero for a period of one millisecond will extinguish the subsequent discharge in the main gap. The main gap can then be refired by a trigger pulse as above and extinguished again in a repetitive cycle at a repetition frequency of the order of 900 periods/ second.

While the principles of the invention have been described above in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. A cold cathode gas filled electric discharge tube comprising an anode opposed to a single unitary cathode having an activated discharge surface of extended area forming with said anode a main discharge gap and trigger electrode adjacent the said cathode forming therewith an auxiliary discharge gap of lower striking voltage than the said main gap, wherein the said cathode is annular and the said trigger electrode is positioned within the enclosure of the cathode, the auxiliary discharge gap being substantially at right angles to and adjacent the discharge path of the main gap.

2. A discharge tube according to claim 1 in which the said anode is a metal rod mounted end-on to the said cathode and in which the said trigger electrode is formed to present an extended surface opposed to the inner edge of the cathode.

3. A discharge tube according to claim 1 in which the three said electrodes are mounted on respective rods sealed in a glass press, the discharge surface of the said anode being intermediate the said press and the said cathode, said cathode being coated with emissive material as the underside thereof.

4. A cold cathode gas filled electric discharge tube comprising a glass envelope, a glass press at one end of the said envelope, three metallic rods sealed in line in the said press and a single unitary annular cathode mounted on an outer one of the said rods overlapping the other outer said rod and coated with electron emissive material on the side facing the said other outer rod, and a metal strip secured to the middle said rod and shaped with a broad side coaxially opposed to the inner edge of the said cathode along an arc thereof to either side of the said other outer rod.

5. A discharge tube according to claim 4 in which the outer and inner edges of the cathode annulus lie on eccentric circles, the cathode being mounted so that the said anode or the said other outer rod, respectively is directed towards the widest part of the cathode.

6. A discharge tube according to claim 4 in which the said cathode is an annular disc, the surface adjacent the inner edge thereof being bevelled on the side facing the said anode or, respectively, the said other outer rod.

7. A discharge tube according to claim 4 in which the rod mounting the said trigger electrode, or the said middle rod, respectively, projects through and beyond the cathode to provide a mount for a getter assembly on the side of the cathode remote from the said press.

References Cited in the file of this patent UNITED STATES PATENTS 2,331,398 Ingram Oct. 12, 1943 

